It is known that by heating compounds having the formula P.sub.2 NXCl.sub.5, wherein X designates an oxygen or sulfur atom, under conditions of pressure and temperature adequate to release PXCl.sub.3, there are formed linear polychlorophosphazenes having a terminal group --PXCl.sub.2. Thus, in French Pat. No. 79 24037 (publication No. 2466435) there is described the preparation of linear polychlorophosphazenes having a terminal group dichlorophosphoryl of the formula --POCl.sub.2 by polycondensing the compound P--trichloro N-dichlorophosphoryl monophosphazene of the formula P.sub.2 NOCl.sub.5 by heating under pressure and temperature conditions which release POCl.sub.3. In French patent application No. 83 11264 of July 6, 1983 (publication No. 2548652) in applicant's name there is described the preparation of linear polychlorophosphazenes having a terminal group dichlorothiophosphoryl of the formula --PSCl.sub.2 by polycondensing the compound P-trichloro N-dichlorothiophosphoryl monophosphazene of the formula P.sub.2 NSCl.sub.5 by heating under pressure and temperature conditions to cause release of PSCl.sub.3.
The linear polychlorophosphazenes formed have a terminal group --PXCl.sub.2 and correspond to the formula: EQU Cl.sub.2 (X)P[NPCl.sub.2 ].sub.n Cl (I)
wherein n is a number equal to or greater than 4. It is possible to control the value of n to obtain chains of medium length.
Thus, n can be from 4 to 1000, for example, or even up to values as high as 5000 or more.
Without being bound by this hypothesis, it is thought that the polycondensation of P.sub.2 NXCl.sub.5 the formula of which can be represented are: ##STR1## is carried out according to the reaction diagram: ##STR2## Formula (II) represents the linear polychlorophosphazenes having the formula (I).
Despite undeniable advantages and the simplicity of the process, the high rate of conversion of the monomer and the relatively low cost of said monomer, the above cited process for preparation of linear polychlorophosphazene with a terminal group --PXCl.sub.2 by polycondensation of the P.sub.2 NXCl.sub.5 monomer, by heating, is not entirely satisfactory inasmuch as it is not easy to obtain a high degree of polycondensation in a reproducible manner.
In fact, when the release of PXCl.sub.3 stops, the medium degree of polycondensation of the linear polychlorophosphazene formed is low, generally, on the order of n of from about 20 to 30, which means that the polychlorophosphazene products are of short chain length. To obtain a polycondensation product having a higher degree of polycondensation, the products of short chain length must react between themselves by their ends to form products of long chain length, which requires that the polycondensation be continued after the termination of the release of the PXCl.sub.3 compound. The course of this second phase of the polycondensation is very hard to control and the production of uncrosslinked polychlorophosphazenes having a high degree of polycondensation that is above about 100 is difficult to attain. In fact, the risk of cross-linking increases with the increase in the length of the polymer chains and this phenomenon develops in a manner that changes from one batch to the other. Therefore, what results is poor reproducibility when it is sought to obtain high degrees of polycondensation. This constitutes a serious handicap for industrially carrying out the process, since most of the application of the linear polychlorophosphazenes call for products of high molecular weight, that is, products having a high degree of polycondensation.
The invention provides an improvement on the above cited process for preparation of linear polychlorophosphazenes with --PXCl.sub.2 terminal groups which makes it possible to minimize the cross-linking phenomena in the course of the polycondensation and thus to obtain in a reproducible manner high degrees of polycondensation.